Dynamic residual complexity of natural products by qHNMR: solution stability of desmethylxanthohumol

The use of chromatographic assays to assess the residual complexity of materials that are purified from natural sources by chromatographic means is, in a sense, a case of the fox watching the henhouse. Beside their static residual complexity, which is intrinsic to their metabolic origin, biologically active natural materials can also be involved in chemical reactions that lead to dynamic residual complexity. The present study examines the dynamics of the hop prenylphenol, desmethylxanthohumol (DMX), by means of quantitative (1)H-NMR (qHNMR) in a setting that mimics IN VITRO and physiological conditions. The experiments provide a comprehensive, time-resolved, and mechanistic picture of the spontaneous isomerization of DMX into congeneric flavanones, including their (1)H/(2)D isotopomers. Formation of the potent phytoestrogen, 8-prenylnaringenin (8PN), suggests that measurable estrogenic activity even of high-purity DMX is an artifact. Together with previously established qHNMR assays including purity activity relationships (PARs), dynamic qHNMR assays complement important steps of the post-isolation evaluation of natural products. Thus, qHNMR allows assessment of several unexpected effects that potentially break the assumed linkage between a single chemical entity (SCE) and biological endpoints.

Localization of D-β-aspartyl residue-containing proteins in various tissues

Prior to the emergence of life, it is believed that only l-amino acids were selected for formation of protein and that d-amino acids were eliminated on the primitive Earth. Whilst homochirality is essential for life, the occurrence of proteins containing d-β-aspartyl (Asp) residues in various tissues from elderly subjects has been reported recently. Here, we demonstrate the presence of a d-β-Asp-containing protein in the cardiac muscle of heart, blood vessels of the lung, chief cells of the stomach, longitudinal and circular muscle of the stomach, small intestine and large intestine. Since the d-β-Asp residue occurs through a succinimide intermediate, this isomer may potentially be generated in proteins more easily than initially thought. Formation of the d-β-Asp residue in proteins may be related to stress.

Formation of stable and metastable porphyrin- and corrole-iron(IV) complexes and isomerizations to iron(III) macrocycle radical cations

Oxidations of three porphyrin-iron(III) complexes (1) with ferric perchlorate, Fe(ClO(4))(3), in acetonitrile solutions at -40 degrees C gave metastable porphyrin-iron(IV) diperchlorate complexes (2) that isomerized to known iron(III) diperchlorate porphyrin radical cations (3) when the solutions were warmed to room temperature. The 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) systems were studied by UV-visible spectroscopy. Low temperature NMR spectroscopy and effective magnetic moment measurements were possible with the TPP and TMP iron(IV) complexes. Reactions of two corrole systems, 5,10,15-tris(pentafluorophenyl)corrole (TPFC) and 5,15-bis(pentafluorophenyl)-10-p-methoxyphenylcorrole (BPFMC), also were studied. The corrole-iron(IV) chlorides reacted with silver salts to give corrole-iron(IV) complexes. The corrole-iron(IV) nitrate complexes were stable at room temperature. (TPFC)-iron(IV) toslyate, (TPFC)-iron(IV) chlorate, and (BPFMC)-iron(IV) chlorate were metastable and rearranged to their electronic isomers iron(III) corrole radical cations at room temperature. (TPFC)-iron(III) perchlorate corrole radical cation was the only product observed from reaction of the corrole-iron(IV) chloride with silver perchlorate. For the metastable iron(IV) species, the rates of isomerizations to the iron(III) macrocycle radical cation electronic isomers in dilute acetonitrile solutions were relatively insensitive to electron demands of the macrocyclic ligand but reflected the binding strength of the ligand to iron. Kinetic studies at varying temperatures and concentrations indicated that the mechanisms of the isomerization reactions are complex, involving mixed order reactivity.

Biosynthetic potential of sesquiterpene synthases: alternative products of tobacco 5-epi-aristolochene synthase

Nicotiana tabacum (tobacco) 5-epi-aristolochene synthase (TEAS) serves as an useful model for understanding the enzyme mechanisms of sesquiterpene biosynthesis. Despite extensive bio-chemical and structural characterization of TEAS, a more detailed analysis of the reaction product spectrum is lacking. This study reports the discovery and quantification of several alternative sesquiterpene products generated by recombinant TEAS in the single-vial GC-MS assay. The combined use of chiral and non-polar stationary phases for gas chromatography separations proved critical for resolving the numerous sesquiterpene products of TEAS for mass spectral analysis and identification. Co-injection studies with available authentic standards from both synthetic and natural sources further corroborated the assignment of several compounds, resulting in an annotated reaction mechanism accounting for their biosynthesis. Moreover, a previously undocumented farnesyl trans-cis isomerization pathway was observed.

Enantioselective Incorporation of Azobenzenes into Oligodeoxyribonucleotide for Effective Photoregulation of Duplex Formation

A drop in melting point of 21.5°C is induced by the UV-photolytic trans→cis isomerization of the duplex formed between an oligonucleotide bearing two D-threoninol-tethered azobenzene moieties in the side chain and its complementary counterpart. On irradiation with visible light, the dissociated single-stranded oligonucleotides regenerate the duplex.

Effects of Infrared Laser Radiation on the In Vitro Isomerization of All-Trans Retinal to 11-Cis Retinal

The in vitro effect of infrared laser light on the isomerization of all-trans retinal dissolved in an ether/hexane and also an ethanol solvent was studied. Pulsed laser energy at 1064 nm was used to drive the molecular reconfiguration of all-trans retinal to 11-cis retinal. High pressure liquid chromatography (HPLC) was used to quantify the conversion. Overall isomerization was minimal (0.2 percent to 1.0 percent), yet, a significant difference in isomerization due to pulsed infrared laser energy over non-modulated monochromatic laser light was detected (up to 168 percent difference). Potentially, pulsed laser radiation tuned to the ethylenic stretch frequency of the C11=C12 bond of retinal may induce rotational changes to the chromophore.

The Enthalpies of Combustion and Formation of Ortho- and Parafluorobenzoic Acid

The enthalpies of combustion and formation of one sample of ortho- and two samples of parafluorobenzoic acids have been determined by combustion in an oxygen-bomb calorimeter. The data obtained by other investigators are discussed briefly. The values obtained and their estimated uncertainties are as follows: [Table: see text] where ΔHc° corresponds to the reaction: C 7 H 5 O 2 F ( c ) + 7 O 2 ( g ) + 48 H 2 O ( liq ) → 7 CO 2 ( g ) + [ HF + 50 H 2 O ] ( liq ) . .

The Enthalpies of Combustion and Formation of the Mono-chlorobenzoic Acids

The enthalpies of combustion of o-, m-, and p-chlorobenzoic acid have been determined in an adiabatic rotating-bomb calorimeter. The enthalpies of formation have been obtained by combination of the experimental data with the accepted values for the enthalpies of formation of water, carbon dioxide, and aqueous hydrochloric acid. The results of other investigators are discussed briefly. The resulting values and their estimated 95 percent confidence limits are as follows: o-Chlorobenzoic acidm-Chlorobenzoic acidp-Chlorobenzoic acid ΔHc° (25 °C)-3087.91 ± 0.69 kJ/mol-3068.05 ± 1.53 kJ/mol-3064.40 ± 0.66 kJ/molΔHf° (25 °C)-404.83 ± 0.74 kJ/mol-424.59 ± 1.55 kJ/mol-428.16 ± 0.72 kJ/mol. Where ΔHc° corresponds to the process: -404.61-424.37-429.94C7H5O2Cl(c) + 7 O2(g) + 198 H2O(liq) → 7 CO2(g) + [HC1 + 200 H2O](liq).

Photoionization of C 4 H 8 + Isomers. Unimolecular and Bimolecular Reactions of the C 4 H 8 + Ions

1-Butene, cis-2-butene, isobutene and methylcyclopropane have been photoionized with the resonance lines of krypton (10.0-10.6 eV) and argon (11.6-11.8 eV). We have determined that the internally excited 1 - C 4 H 8 + ion and, to a much lesser extent, the i - C 4 H 8 + ion isomerizes to the 2 - C 4 H 8 + structure. In both cases the extent of isomerization increases, approximately by a factor of ten when the photon energy is increased from 10 to 11.7 eV. An inert gas, neon, quenches the isomerization of the i - C 4 H 8 + ion and, to a much lesser rextent, that of the 1 - C 4 H 8 + ion. The unimolecular fragmentation of theC 4 H 8 + isomeric ions has been examined at 11.61-11.8 eV. In this energy range the dissociative lifetime of i - C 4 H 8 + was found to be at least 5 × 10-6 s, and collisional quenching of the dissociative process is already noticeable at pressures in the 10-3 torr range. The rate coefficients for the reactionC 4 H 8 + (  thermal  ) + C 4 H 8 → ( C 8 H 16 + ) * occurring in the isomeric C4H8 systems have been determined under conditions where the structure of the reactingC 4 H 8 + ion is established. The values in cm3/molecule · second are 1-C4H8 - 6.0 ± 0.5 × 10-l0, cis-2-C4H8 - 0.37 ± 0.1 × 10-10, i-C4H8 - 5.4 ± 0.4 × 10-10. At pressure below 10-3 torr, the internally excited( C 8 H 16 + ) * produced in the reaction dissociates along various channels with relative probabilities depending upon the structure of both the ionic and neutral reactant. Above 10-3 torr collisional quenching of( C 8 H 16 + ) * is noted.

Isomerization Processes in Ions of the Empirical Formula C 4 H 8 +

Ions of the formulaC 4 H 8 + have been generated with different initial energies by ionizing ethylene ( C 2 H 4 + + C 2 H 4 → C 4 H 8 + , where theC 4 H 8 + ion is formed with an initial energy of > 11.51 eV), cyclobutane (initial energy ofC 4 H 8 + , > 10.84 eV), methylcyclopropane (> 10.15 eV), 1-C4H8 (> 9.58 eV), and i-C4H8 (> 9.06 eV) with 11.6-11.8 eV photons, and in some cases also with 10 eV photons and with gamma radiation. The structures of the ions have been determined from the structures of the C4H8 products formed in charge transfer reaction between the ions and charge acceptors such as dimethylamine and nitric oxide, as well as from the structures of the butanes formed inD 2 - transfer reactions with methylcyclopentane-d 12 (C 4 H 8 + + C 6 D 12 → C 4 H 8 D 2 + C 6 D 10 + ). At low pressures theC 4 H 8 + ions initially formed in ethylene, cyclobutane, and methylcyclopropane isomerize to the thermodynamically most stable configurations,i-C 4 H 8 + and2-C 4 H 8 + . The2-C 4 H 8 + structure predominates in all the experiments. As the pressure is raised, thei-C 4 H 8 + ion yield diminishes as that of2-C 4 H 8 + increases, indicating that when the precursor of thei-C 4 H 8 + ion is collisionally deactivated, it ends up as2-C 4 H 8 + . At high pressures,1-C 4 H 8 + ions are intercepted; their yield increases with increasing pressure, indicating that1-C 4 H 8 + is an intermediate which isomerizes further unless it is collisionally deactivated. The1-C 4 H 8 + ion formed in methylcyclopropane (initial energy > 10.15 eV) is more easily deactivated than that formed in cyclobutane (initial energy > 10.84 eV). That the isomerization of the1-C 4 H 8 + ion to lower energy structures such asi-C 4 H 8 + and2-C 4 H 8 + requires excess internal energy is demonstrated by the fact that in the photolysis with 10 eV photons, a negligible amount of isomerization is observed, but with 11.6-11.8 eV photons, more than half of the1-C 4 H 8 + ions isomerize to the2-C 4 H 8 + structure at a pressure of 2 torr. Isomerization of the low energyi-C 4 H 8 + ions formed in the photolysis of i-C4H8 to other structures is relatively unimportant at 11.6-11.8 eV. Taking the ratioi-C 4 H 8 + / 2 -C 4 H 8 + as an indicator of the amount of energy removed by collisions from the intermediateC 4 H 8 + species under conditions where only i- and 2 -C 4 H 8 + ions are intercepted, it is shown that the efficiency of energy transfer from the ions to helium, hydrogen, neon, krypton, xenon, nitrogen, and carbon dioxide is related to the polarizability of the added deactivator.